Reduce minimum width of Zotero pane to 670px

We could potentially do more, but this is the narrowest the pane can be
with the left pane collapsed without starting to hide toolbar icons, so
let's try this. (This will still hide the sync icon if the left pane
isn't collapsed.)

Closes #675
Closes #1183

chrome/skin/default/zotero/standalone.css

@@ -3,6 +3,6 @@
 }
 
 #main-window {
-	min-width: 915px;
+	min-width: 670px;
 	min-height: 300px;
 }